This section provides background information related to the present disclosure which is not necessarily prior art.
Welding robot systems having spot welding guns are described, for example, in U.S. Pat. Appl. Pub. No. 2011/0089146 A1 to Takahashi et al. and U.S. Pat. No. 5,898,285 to Nagasawa et al. A typical spot welding servo gun includes a main body having a stationary electrode tip and a movable electrode tip. The stationary electrode tip is disposed opposite the movable electrode tip. The stationary electrode tip is generally immovable relative to the main body of the spot welding gun, and the movable electrode tip is mounted on the main body and opened and closed during a welding operation.
U.S. Pat. No. 9,144,860 B2 to Garza et al. describes a method for controlling a welding robot having a servo gun with a movable tip and a fixed tip including the steps of separately contacting a surface of a part with the movable tip to measure a base test point and an at least two additional test points displaced from the base test point. Two non-collinear vectors are calculated from the at least two additional test points. A normal vector is calculated from the two non-collinear vectors. An angle between the original servo gun orientation and the normal vector is determined. An orientation of the servo gun may be corrected, for example, if the angle is within user specified tolerances.
Another issue related to controlling a servo gun is the pressing force. Japanese Patent Publication No. 10-94882 discloses a method for controlling a pressing force of a welding gun. In the method, a small quantity of an elastic displacement, generated in a fixed side electrode tip when a moving side electrode tip is driven to contact the fixed side electrode tip and further driven to press the fixed side electrode tip, is detected by an encoder of a servo motor for driving the moving side electrode tip. More particularly, the elastic displacement is determined based on the rotation increment of the servo motor generated from the time when the moving side electrode tip begins to contact the fixed side electrode tip to when the electric current suddenly increases when the moving side electrode tip is further driven to press the fixed side electrode tip. A real pressing force between the electrode tips is calculated based on the measured elastic displacement of the fixed side electrode tip. Then, a setting pressing force between the electrode tips is modified to be equal to the calculated real pressing force.
However, there are the following problems with the above-described conventional method for controlling a welding gun.
First, since a moving side portion of the welding gun including the moving side electrode tip and the servo motor for driving the moving side electrode tip is provided with a speed reducer, a mechanical impedance of the moving side portion is greater and dynamically harder than that of a fixed side portion of the welding gun. In this instance, the mechanical impedance is defined as an impedance expressed by a vector of |m, c, k|, when a movement of the electrode tip is expressed by an equation:m·d2x/dt2+c·dx/dt+kx=F(t).
In a case where the vector only includes k, the mechanical impedance is a spring constant. The encoder is located on the opposite side of the moving side electrode tip with respect to the speed reducer of the servo motor, so that a pressing quantity of the moving side electrode tip, transmission of a change in the pressing displacement and the pressing force to the encoder through the speed reducer is a small amount and is delayed. As a result, the responsibility is lowered, so that it is difficult to obtain an accurate response with the conventional method, in which the welding gun is controlled based on an output of the encoder.
Second, since the rigidity of an arm supporting the fixed side electrode tip is increased so that the mechanical impedance of the fixed side portion including the fixed side electrode tip and the arm is nearly equal to the mechanical impedance of the moving side portion, the size and the weight of the arm is large, which causes the welding gun to be increased both in size and weight.
U.S. Pat. No. 6,531,674 B2 to Suita describes an intelligent welding gun provided with a fixed side sensor in a fixed side portion. The mechanical impedance of the fixed side portion is kept small, which permits the mechanical impedance to be set in a range where the fixed side sensor can effectively detect at least one of a position of a fixed side welding tip and a pressing force imposed on the fixed side welding tip. The fixed side sensor and a moving side sensor constitute a redundant sensor measurement system. Various kinds of methods conducted using the above welding gun include a method of calibrating a sensor (including calibration of a reference point and a gain), a control method of suppressing a welding expulsion, a re-welding feedback control method, a control method of a welding strength, a control method of reducing a clearance between workpieces, a method of correcting a welding robot track, and a method of managing a positional accuracy change at a welding point.
However, the welding gun with sensors described in U.S. Pat. No. 6,531,674 B2 also has shortcomings. This welding gun is designed for adaptation; not for defect detection. Also, this welding gun requires sensors.